Green-Ampt Parameterization for Rainfall Excess
Determination
Harry Downing
SWFWMD
April 18, 2008
Green-Ampt (GA) Parameter Outline
• The District has changed to GA?• GA Process (General)• Data Sources for Developing GA Parameters• Look-Up Table Development (Soils and LU)• ArcHydro Tools for GA• Parameter Generation
• Questions
Rainfall Excess Process
Soil Parameter Sources
• NRCS - ‘Soil Survey’ and Soil Survey Geographical (SSURGO) Database - Depth to Water Table and Ksat
• IFAS - ‘Site Specific Soil Characterization’ – Soil Moisture Retention Information
• Table Information from Other Studies (Text Books)
• Site Specific Characteristics from ERP Applications
Soil Survey Information
Soil Survey Information Continued
Soil Moisture for 200 cm Depth to WTH
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Soil Moisture Content %
Soil Moisture
Saturation / Soil Porosity
Soil Look-Up Table
Land Use Look-up Table
Creative Land Use Look-Up Table
ESRI ArcHydro Tools
ESRI Sub-Catchment Assignment of GA Parameters
Questions?
Infiltration and Percolation Parameterization
Take You Through Process that the District has Used
Parameters Generation
1. Area Of Unsaturated and Saturated Infiltration
2. Perimeter Development
3. Hydraulic Properties of the Soil
4. Expected Depth to the Water Table (SHGWT or Higher)
5. Development of Parameters for Input Into ICPR
Area of Infiltration
Area of Unsaturated Infiltration
• First Make a Run with the GA Excess Option Only
• Obtain the Maximum Inundation Area and Elevation from the Model Results
• Develop a GIS feature class of the Inundation areas (area & perimeter) - units
• Use this Information for the Infiltration and Percolation Parameters for ICPR regarding initial area
Hydraulic Properties of Soils
• SSURGO and IFAS Data• ERP Information• Geotechnical Information Reports• Soil Testing
Location of Soil Bores Within Hernando County
Vertical Hydraulic Conductivity Results
Results from ERP Tests
SSURGO Results for Hydrologic ‘A’ Soils
• Minimum Value 0.028 inches/hr
• Mean 21.25
• Max 35 inches/hr
Assumptions
• Horizontal Hydraulic Conductivity Greater Than Vertical• Use of Vertical for Horizontal Should be Conservative
Depth to SHGWT
• Use SSURGO
• Potentiometric Surface Map
• Land Use Map
• Imagery
ICPR Parameterization
ICPR Parameterization Continued
Questions?
Floodplain ResultComparisons
Stage Plot Comparison at Node
Stage Plot Comparison at Node
Stage Plot Comparison at Node
Reasonableness of Floodplain
ITEMS TO CHECK !!
• Double Accounting - Between GA, and Infiltration and Percolation
• Area of Influence Around Infiltration Areas
Questions?